The Long Term Evolution (LTE) series of standards defines an Evolved Packet System (EPS) mobility management (EMM) sublayer to support user equipment mobility. EMM also provides connection management services to the session management sublayer and the short message services entity of the connection management sublayer. EMM connection management procedures include the service request, the paging procedure, and the like. EMM procedures are typically performed when a non-access stratum (NAS) signaling connection has been established between the user equipment and the network; otherwise, the EMM may initiate establishment of a NAS signaling connection.
The EMM's service request procedure is typically invoked when one or more of the following occur: the network has downlink signaling pending; the user equipment has uplink signaling pending; the user equipment or the network has user data pending and the user equipment is in an idle mode (for example, EMM-IDLE); the user equipment, in an idle mode (for example, EMM-IDLE) or connected mode (for example, EMM-CONNECTED), has requested to perform a mobile originating/terminating circuit switch (CS) fallback or 1xCS fallback; the network has downlink cdma2000® signaling pending; or the user equipment has uplink cdma2000® signaling pending. Moreover, the service request procedure may be initiated by the user equipment for the downlink transfer of signaling, CDMA 2000 signaling, or user data in EMM-IDLE mode, in which case the network may trigger the user equipment via a paging procedure.
If a service request is not accepted by the network, the network may send a service reject message to the user equipment. Moreover, a node in the network, such as a mobility management entity (MME), may define a reason for rejecting the service request from the user equipment. For example, if a service request from a user equipment cannot be accepted, the network may return a service reject message to the user equipment, and the service reject message may include a cause code for the rejection.